192 
cause for the marvellously perfect cases of adaptive 
resemblance so common among insects. 
Apropos of the extermination of plants, Dr. Abbott 
remarks (p. 41) with respect to the witch-hazel (Hama- 
melis virginica) :—“ Bent twigs of this plant are still 
used by the ‘gifted’ to find water, lost farming tools, 
and, by one enthusiast, Indian graves. The faithful still 
claim it as efficacious, and he who doubts is sneered at if 
he expresses his opinion. All that the rambler can ask is 
that the plant be not exterminated, and that the fools 
may be.” We may perhaps echo this sentiment on this 
side of the Atlantic without offence to the members of 
“Primrose” or any other floral “ Leagues.” A protest 
against the extermination of rare plants by “dealers ” 
was circulated by the Corresponding Societies Committee 
of the British Association last year. 
We have given a sufficient idea of this work to commend 
it to the notice of English naturalists, and we may remark 
in conclusion that, although the animals and_ plants 
referred to are not familiar to the ramblers by our own 
streams, the sparkling anecdotal style will cause the 
volume to be enjoyed by all, whether trained observers or 
casual country wanderers, and the spirit in which the 
author goes forth into the fields and woods or saunters by 
his favourite “‘ Poaetquissings ” may be well imitated by 
the numerous field naturalists now being called into 
activity by the widely-spread establishment of local 
societies. ‘He who has this interest in the life about 
him can never be lonely, wander wheresoever he will, nor 
return from a contemplative ramble other than a wiser 
and happier man.” R. M. 
CEDITERS TO THE SED) TOK 
[The Editor does not hold himself responsible for opinions ex- 
pressed by his correspondents. Neither can he undertake to 
return, or to correspond with the writers of, rejected manu- 
scripts, No notice is taken of anonymous communications. 
[The Editor urgently requests correspondents to keep their letlers 
as short as possible. The pressure on his space is so great 
that it is impossible otherwise to insure the appearance even 
of communications containing interesting and novel facts. | 
On Refractometers 
I OBSERVE that in your issue of June 17 (p. 157) there is an 
article by Mr. Gordon Thompson on ‘‘‘the Determination of 
the Index of Refraction of a Fluid by Means of the Microscope.” 
The method there described was, I believe, first proposed by 
the Duke de Chaulnes in 1767 ; and in 1876 was suggested by 
Dr. Royston Pigott in connection with his refractometer. It was 
employed in 1878 by Dr. Sorby for recognising the minerals in 
thin slices of rocks ; and in 1884 by Dr. Bleekrode in determin- 
ing the refraction of liquefied gases (/voc. Royal Society, vol. 
XXXVil. p. 343). In these two instances the ordinary method 
was unavailable. The proposed method has not been much 
used owing to the fact that the index of refraction cannot be at 
all depended upon beyond the third place of decimals. 
Mr. Thompson considerably exaggerates the difficulty of the 
usual method by means of a hollow prism: the angle of the 
prism may be determined once for all; the position of minimum 
deviation presents no difficulty ; and the use of monochromatic 
light is unnecessary. Indeed it would be objectionable, as it 
would prevent the determination of the dispersive power, which 
is often of equal importance with the refractive power of the 
substance. In my own experiments I have often taken observa- 
tions both of the refraction and dispersion of five or six liquids 
during the course of an hour, including the cleaning of the prism 
between each. 
The suggested method seems scarcely to admit of determining 
the temperature of the drop with any accuracy, which is an im- 
portant matter where liquids are concerned. It may, however, 
NATURE 
[Fuly 1, 1886 
doubtless be employed by those who have a good microscope, 
where great accuracy is not required. 
There is an instrument called Abbe’s refractometer, which I 
have recently used for preliminary determinations, and I find it 
gives accurate results to the third place of decimals. It is 
founded on the principle of total reflection, It requires also 
only a drop of the liquid, and as the index of line D is read off 
without any calculation a complete determination can be made 
in a minute or two. ‘There is also an arrangement by which the 
dispersion D to F can be observed and calculated, but I do not 
find that this is accurate enough to be of much service, The 
instrument is to be obtained of Carl Zeiss of Jena. 
17, Pembridge Square, June 26 J. H. GLADSTONE 
Luminous Boreal Clouds 
DURING the past two or three years what appears to the 
writer a distinct class of luminous night clouds in the north sky 
have occupied his attention. They have probably not escaped 
more competent observers, and been perhaps referred to simple 
auroral phenomena, thus escaping discussion. A very marked 
example was visible here the night before last (22nd inst.), of 
which inclosed is an illustration from a sketch at the moment. 
I may premise the sky was generally clear, stars bright, tem- 
perature very low, and wind strong (N.B.) from north-west—a 
direction maintained for the past two days. Only aslight degree 
of illumination was imparted to the clouds by a low moon in the 
south-east, near last quarter. Some light cirrus ‘‘scud,” high 
up, conformed to direction of wind. 
Above and behind a dark but very limited bank of strato- 
cumulus, a luminous cloudlet of brilliant pearly Justre appeared, 
nvt concurrent exactly with either the magnetic or true meri 
dians, in altitude from 5° to 10° from the horizon, and for 7° in 
horizontal arc. Its shape, character, and position little varied 
during observation from 11.30 p.m. to2a.m. The structure in 
this case (only partially realised in the sketch) was striated, the 
“strike” of main streaks being north-east and south-west. 
Transverse bars of luminosity conformed closely to the direction 
of the cirrus clouds above, and of the wind. On the three or 
four other occasions of such observations these luminous cloud- 
lets have been devoid of structure, but in every case they have 
presented, as in this, an opaque pearly lustre, with definite 
outline. 
Of an entirely different type to the eye are the sudden, diffuse, 
variable, and transient transparencies of aurorze. Avoiding pre- 
mature discussion, one cannot but suspect the former occur in 
much lower and less rare sky-tracts probably than the latter, 
with a possible frictional factor in their development ; and might 
be distinguished as 2ud.cul@ boreals if accorded a special place 
on further observation. The temperature has been keeping low, 
and sunset after-glows have in some degree reappeared during 
the past week ; especially gorgeous being the cloud-tints at sun- 
set of the 22nd inst. D. J. Rowan 
Dundrum, co. Dublin, June 24 
Ampére’s Rule 
WITH regard to Ampere’s rule I should be glad to know what 
is the general experience of actual teachers ? “4 
I have taught electricity to boys for four years, and when at 
Rugby I learned the subject for I think two years. 
ence has been that ‘‘ Ampére’s rule ” is 7zof confusing ; and as a 
teacher I find it best to give both this rule and the ‘‘screw- 
motion” rule. I see that Mr. Cumming gives both, on p. 222 
of his book. W. L. 
The College, Cheltenham 7 
As Prof, Daehne (NATURE, June 24, p. 168) has called 
attention again to the treatment of Ampére’s rule in my 
‘Electricity Treated Experimentally,” perhaps you will allow 
me to point out that the rule given by Ampere is quoted /i- 
torically only, and for it is substituted a rule, due, I believe, to 
Clerk-Maxwell, which seems to me preferable to either the 
original rule of Ampere, or to that quoted by Prof. Daehne, 
namely, that the movement of a north pole is right-handed to 
the direction of the current. That is to say, if we assume any 
right-handed screw to be propelled along the current, the nortl 
pole will move in the direction of the twist in the muscles of th 
wrist in propelling it ; and vice versa, if the north pole moved 
¥ 
: 
My experi- | 
